Exothermic welding compositions are employed to produce a wide variety of electrical and mechanical joints. Such compositions and the apparatus for using such compositions are sold by Erico Products Inc. of Solon, Ohio, U.S.A. under the trademark CADWELD.
In the process of use, typically, a graphite mold which includes a crucible encloses the parts to be joined. A weld cavity surrounds the parts to be joined and is connected to the bottom of the crucible by a tap hole. One or more steel disks close the bottom of the crucible. Exothermic material is then placed in the crucible and is covered by a layer of low ignition starting material. When the crucible is closed the starting material is ignited through the use of a spark ignitor. The starting material in turn starts the exothermic reaction which converts the material to molten metal and slag which rises to the top. The molten metal then melts through the disk and permits the molten weld metal to pass through the tap hole to fill the weld cavity.
In some situations it is desirable to remotely ignite the mixture. Also, the handling, shipping and preparatory process for the use of the starting material makes desirable an exothermic ignition process which would avoid the use of the starting material.
For years exothermic materials have been ignited by a hot wire process. Such hot wire process has several drawbacks. One of those drawbacks is reliability. It has been found that in order for ignition to occur, the wire must transfer enough heat to the material before it fuses. If the wire fuses and ignition doesn't occur then the wire has to be replaced, reconnected and properly positioned in the exothermic material. Although coils, lengths, or other patterns may be employed throughout the mixture such as seen in French patent specification No. 324,534 to Weber, because of the fusion of the wire, ignition, if it occurs at all, will normally occur only at a single point in the mixture. In some exothermic processes single point ignition may be acceptable. However, to produce a quality weld random single point ignition is not acceptable.
Other attempts at electrical ignition have been made, one utilizing highly esoteric and, of course, expensive solid rocket igniters such as shown in the copending application of Dennis Brosnan et al entitled "Exothermic Welding Apparatus and Method", filed Mar. 25, 1987, U.S. Ser. No. 030,169. Such igniters, unless modified or duplicated, again expensively, provide only single point ignition.
In conventional welding applications as indicated the starting material is spread over the top of the weld metal which is contained in the crucible section of the mold. When the starting material is ignited the exothermic material is ignited at the top and the reaction moves downwardly forming such molten metal and slag. When the molten metal reaches the disk and sufficiently elevates its temperature, the disk melts and the molten metal drops through the tap hole to the weld cavity.
In a proper process a delay occurs between ignition and the dropping of the molten metal. Such delay results in completion of the reaction for all of the mixture and also permits any slag formed to rise to the top of the molten metal. Thus any slag formed will be drawn into the weld cavity last and will collect in a riser which may subsequently be removed. Thus if the disk melts through prematurely, slag or unreacted exothermic material or both may be drawn into the critical portion of the weld cavity, adversely affecting the quality of the weld.
In addition to the above outlined problems of hot wire remote electrical ignition of exothermic materials, there is the problem of the power source. Many exothermic welding procedures are conducted in the field where portable electrical power may not readily be available. In the past automobile batteries have been used for such purposes but such batteries are really not readily portable. Moreover, such batteries and circuits don't provide reliable ignition.
It is therefore desirable to provide a reliable low cost remote electrical ignition system for exothermic materials which does not require starting material, and one which utilizes a lightweight and hand portable power source. Moreover, it is important that when the process is used to form welds, it is desirable the ignition occur over substantially the top surface of the mixture so that the problems noted with single point ignition are avoided.